Drying a Filter Cake in the Constant-Rate Region. A wet filter cake in a pan 1 ft x 1 ft square and 1 in. thick is dried on the top surface with air at a wet bulb temperature of 80 F and a dry bulb of 120°F flowing parallel to the surface at a velocity of 2.5 ft/s. The dry density of the cake is 120 lb/It3 and the critical free moisture content is 0.09 lb H₂O/lb dry solid. How long will it take to dry the material from a free moisture content of 0.20 lb H₂O/lb dry material to the Ans. t 133 h critical moisture content?